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# Blog Details

## Capacitor as a Charge Well

This is simple experiment I conducted to see effect of capacitor as a charge well/ charge pump to provide current burst in intervals.

I had an old cell phone battery and wanted to use it for running my small alarm clock which works on single AA battery. Now a look at datasheet of battery shows capacity of 1100 mAh. I assumed it lasts for 6 months.

So,
6months=18024hrs=4320hrs6months=180∗24hrs=4320hrs

which means, clock requires a current of
I=1100/4320mA=255μAI=1100/4320mA=255μA

The cell phone battery has voltage of 3.2V3.2V. Instead of putting clock in parallel with a voltage divider, I put it in serial with a potentiometer of 5kΩ5kΩ.

Now I turned the pot to find maximum resistance at which clock still runs. It turned out to be 1kΩ≈1kΩ. But just the resistance with battery would draw 3.2mA≈3.2mA. Which could mean:

1. Clock has huge resistance of 11.5kΩ11.5kΩ or,
2. Clock has some lower resistance, but need intermittent current bursts.

Second option above looks more likely since clock involves mechanical movement after each second.

This gives a good opportunity to verify role of capacitor as charge well. So, initially I added a capacitor of 100μf100μf in parallel with clock. After this addition, clock would run for 454−5 seconds and stop with second hand vibrating in situ.

I thought this might be due to excess capacitance, so replaced the capacitor with 1μf1μf.

Now it started working smoothly. So again I tried to turn the pot till clock stops working. As expected, maximum resistance now increases to 1.9kΩ1.9kΩ.

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